This invention relates to the production of light metal alloys having improved combinations of properties. The invention further relates to a method for making light metal-rare earth metal alloys from pellets of light metal powder and a rare earth metal-containing compound. More particularly, the invention relates to a method for reducing pelletized mixtures of light metal and scandium oxide to form master alloys containing scandium metal.
In the field of alloy development, research is continuously conducted on methods for improving the behavioral characteristics of existing aluminum, magnesium and other light metal alloys. Additional research is directed to the development of new alloy compositions having desired property combinations. Aluminum-based alloys are preferred for many nuclear and aerospace applications because of their relatively high strength-to-weight ratios and corrosion resistance. Magnesium-based alloys possess greater strength-to-weight ratios than most aluminum alloys. These alloys could be made more attractive to manufacturers if it were possible to efficiently and economically incorporate rare earth metals into known or newly developed compositions. That is because even trace amounts of rare earth metals improve corrosion resistance levels and other properties. Minor additions of scandium, for example, are known to improve the tensile and yield strengths of aluminum according to U.S. Pat. No. 3,619,181. Scandium additions of up to about 10% also contribute to the superplastic formability of certain aluminum alloys according to U.S. Pat. No. 4,689,090. Still further improvements may be realized by adding rare earth metals to aluminum brazing alloys (as in U.S. Pat. No. 3,395,001); or by metalliding aluminum surfaces with rare earth metals (as in U.S. Pat. No. 3,522,021). According to Russian Patent Nos. 283,589 and 569,638, scandium additions to magnesium-based alloys improve foundry characteristics, corrosion resistance and/or mechanical strengths.
Although rare earth metal additions improve certain light metal alloy properties, they have not been added to aluminum or magnesium on a commercial scale due, in part, to the difficulty and expense of removing rare earths from the ores containing them. Presently known methods for producing "ingot quality" scandium, for example, require steps for converting scandium oxide to ScF.sub.3 with hydrofluoric acid, reducing the scandium fluoride to a salt, then vacuum melting scandium metal from this salt. This method is rather costly and inefficient, however. About fifty percent (50%) of the scandium within ores treated by this method is not recovered. The "ingot quality" scandium alloy that is produced thereby usually contains minor amounts of titanium and/or tungsten as well. These metals are absorbed by scandium from the special containers used in the aforementioned recovery method.
In U.S. Pat. No. 3,846,121, an alternative method for producing scandium metal was disclosed. This method consists of firing scandium oxide in air to remove any volatile residues therefrom; chlorinating air-fired scandium oxides with phosgene; then reducing the ScCl.sub.3 to magnesium-scandium for subsequent purification by vacuum distillation or arc-melting. Once scandium has been isolated from its ore, it must still be alloyed into one or more metals. Such rare earth metal additions pose their own set of complications. If scandium ingots are directly added to a molten bath of aluminum, scandium aluminide intermetallics tend to form, said intermetallics having melting temperatures hundreds of degrees higher than those associated with aluminum alone. With an increasing presence of these intermetallics, alloy mixing will slow, thereby resulting in an increased chance of producing inhomogenous alloy products.
Several means for directly making light metal-rare earth metal alloys are also known. U.S. Pat. No. 3,855,087, for example, codeposits rare earth metal and aluminum (or magnesium) onto a solid molybdenum, tungsten or tantalum cathode rod by simultaneously reducing oxides of both metals in a molten bath containing LiF and preferred rare earth metal fluorides. The alloy that is produced collects in a non-reactive receptacle placed beneath the cathode rod. In U.S. Pat. No. 4,108,645, a method for making an aluminum-silicon-rare earth metal is claimed which includes reducing rare earth metal oxides with aluminum in the presence of silicon and an alkali metal or alkaline earth metal fluoride flux. The method maintains this flux at a temperature between 1250.degree.-1600.degree. C. West German Patent Application No. 2,350,406 describes a method for producing light metal-rare earth metal master alloys by electrolytically reducing combinations of light metal oxide and rare earth metal oxide in another fluoride salt bath.
In U.S. Pat. No. 3,729,397, there is claimed a method for making magnesium-rare earth metal alloys by reducing rare earth metal oxides in a salt bath with a molten magnesium cathode. After rare earth metal deposits on the cathode confined within a boron nitride sleeve, magnesium-rare earth metal alloy is recovered from this sleeve through ladling, tapping or the like.
French Patent No. 2,555,611 shows a method for reacting rare earth metal oxides with aluminum powder, preferably under an inert gas cover maintained at atmospheric pressure. When a homogeneous mixture of these components is heated at temperatures exceeding 700.degree. C., or well above the melting point for aluminum, an aluminum oxide by-product forms which may be skimmed from the molten alloy surface. In Russian Patent No. 873,692, there is disclosed a method for preparing aluminum-scandium master alloy by combining aluminum powder with scandium fluoride under vacuum in three temperature-increasing stages. This method lowers the fluoride content of the resulting alloy product.
Several means for premixing certain alloying components or subcomponents are also known. U.S. Pat. No. 2,911,297, for example, introduces high melting temperature constituents into molten metal by combining powdered forms of one metal and a salt into a briquette. The salt for this process must be capable of evolving gases at a sufficient pressure for spontaneously disrupting the briquette once it is introduced to the melt. According to the reference, pulverized manganese, copper, nickel or chromium may be added to molten metals by this process.
In U.S. Pat. No. 3,380,820, there is shown a method for making aluminum alloys containing between 2-25% iron. The method includes mixing aluminum with iron particles having a maximum dimension of less than one inch, compressing this mixture into a briquette, and melting the briquette before casting. U.S. Pat. No. 3,503,738 discloses a metallurgical process for preparing aluminum-boron alloys. The process compacts a majority of KBF.sub.4 with finely divided aluminum before adding such compacts to a molten aluminum bath. At least some of the fluoborate in these compacts serves as flux for the reaction.
U.S. Pat. No. 3,592,637 claims an improved process for making direct metal additions to molten aluminum. The process commences by blending finely-divided aluminum powder with one or more other metals selected from: Mn, Cr, W, Mo, Ti, V, Fe, Co, Cu, Ni, Cd, Ta, Zr, Hf and/or Ag. The foregoing blends are then compacted to about 65-95% of their maximum theoretical density. In U.S. Pat. No. 4,648,901, aluminum and another metal component are admixed with a flux of potassium cryolite, potassium chloride, potassium fluoride, sodium chloride, sodium fluoride and/or sodium carbonate before being compacted into "tablets".
In U.S. Pat. No. 3,935,004, recovery efficiencies are enhanced by reducing such aluminum alloying components as manganese, chromium and iron to an average particle size of less than about 0.25 mm before pelletizing these particles with up to 2.5% of a non-hygroscopic fluxing salt and binder, if necessary. U.S. Pat. No. 3,941,588 shows still other means for incorporating materials into molten metal. Such alloying metals as manganese or chromium, for example, may be particulated and admixed with flux and a finely divided phenolic. This mixture is then added to molten aluminum as a powder or in lump, bag or briquette form. In U.S. Pat. No. 4,171,215, finely divided beta manganese particles are blended with aluminum powder before compaction into readily usable briquettes.